Outline

Background: Extra-cranial chordomas require high dose radiotherapy to achieve local control (LC). Recent, univariate analysis of our patients treated with spot scanning based proton therapy (PT) yielded a significantly reduced 5-year LC rate for patients who required prior surgical stabilization (SS) of the axial skeleton compared to patients without SS. This study quantified the effects of SS on local control and dose coverage in patients with extra-cranial chordoma treated with spot scanning based PT.

Methods and Material: Between 1999–2005, 31 patients with Chordoma of C-, T-, L-spine and Sacrum underwent exclusive PT. Eighteen patients had undergone prior surgical stabilization (SS) of the axial skeleton. The mean prescribed dose was 72.7 Gy(RBE). Based on the DVH's of all 31 patients, the following variables were analyzed: prescribed mean proton dose, V95, V80 (% of the volume receiving 95 or 80% of the dose), D98 (dose to 98% of the volume), and gEUD(%) using the method proposed by Niemierko [1] (alpha= –15 for chordomas).

Results: At 5 years, overall LC rate was 48%. Local control was achieved in all 13 patients without SS, resulting in actuarial LC rate at 5 years of 100%. However, 11 local failures (LF) were diagnosed in 18 patients with SS, yielding a 5 year LC rate of 21% (p=0.001). All patients with LF had SS. The prescribed dose was not significantly reduced in patients with SS (p=0.215) or LF (p= 0.433). However, V95 was significantly reduced in patients with SS compared to patients without SS (V95 in pat. with SS: 57.4%; pat. without SS: 74.0%. p= 0.045). In addition, in patients with SS there was a trend towards lower gEUD and V80 values (V80 in patients with SS: 80%; patients without SS: 91%, p= 0.107; gEUD: patients without SS= 83%; patients with SS= 67%, p= 0. 0.085). In patients with LF V95, V80, gEUD and D98 were all somewhat decreased, but not significantly (V95: p= 0.869; V80: p= 0.930, gEUD p= 0.409, D98: p=0.933).

Conclusion: Proton therapy resulted in excellent local control for chordomas of the axial skeleton when no surgical stabilization was present. However, the probability of recurrence of extra-cranial chordomas appears to strongly depend on SS. Based on DVH analysis our results suggests reduced dose coverage (e.g. V95, gEUD) as a result of SS. Due to the limited number of patients it is at present unclear, if presence of SS is an independent variable compared to clinical factors including size of GTV and initial extend of disease.Additional patients are needed to confirm that reduced dose coverage of the PTV is indeed responsible for the reduced LC rate in patients with SS.